1 Technical Field of the Invention
The present invention relates generally to an alternator which may be employed in automotive passenger vehicles or trucks, and more particularly to such an alternator designed to attenuate the field current within a decreased period of time, for example, in the event of a power cable joint failure.
2 Background Art
If a power cable is disconnected from an output terminal of an automotive alternator or the contact thereof is loosened, it may result in surge voltage, thus causing damage to vehicle accessories or semiconductors installed in the alternator.
Specifically, if the power cable is disconnected from the alternator or the contact thereof is loosened instantaneously, and supply of the power to a supply place such as a storage battery is cut, it will cause a no-load saturation voltage to appear at the output terminal of the alternator, thereby resulting in the surge voltage. The no-load saturation voltage continues to be generated as long as a field current is supplied to excite or magnetize field magnetic poles of the alternator.
Typical automotive alternators are driven by an internal combustion engine of the vehicle and used over a wide speed range. The alternators are required to supply an output voltage to vehicle accessories and a battery stably over the wide speed range and so designed as to generate a rated voltage within an idling speed of the engine. For instance, in automotive vehicles designed to have an idling speed of 600 rpm and a speed increasing ratio of 2.5, the speed of the alternator during the idling speed will be 1500 rpm. The alternator is designed to provide an output current of several tens amperes at a rated voltage 14V at 1500 rpm. A power generation starting speed or a rated voltage establishing speed of the alternator is, thus, set to approximately 1000 rpm.
In general, automotive alternators are one of synchronous generators, so that the voltage induced in the armature increases in proportion to the speed of magnetic poles. In automotive alternators designed to develop a rated voltage of 14V at approximately 1000 rpm, the voltage induced in the armature reaches 280V at approximately 20000 rpm that is a maximum serviceable speed. If the power cable is disconnected from the alternator, it will cause a high voltage to appear at an output terminal of the alternator as the no-load saturation voltage.
In order to avoid output of such a high voltage, automotive alternators in recent years have rectifying elements of a full-wave rectifier made of Zener diodes exhibiting the reverse breakdown. However, when the surge voltage is generated in the automotive alternator having the full-wave rectifier made up of Zener diodes, this energy is not emitted out of the alternator, but converted into thermal energy as reverse power consumed in the Zener diodes, thus causing thermal damage to the Zener diodes.
A voltage controller of the automotive alternator is designed to monitor an output voltage of the alternator. When the output voltage exceeds a reference value, the voltage controller cuts supply of field current to decrease a field magnetic flux. Alternatively, when the output voltage is below the reference value, the voltage controller permits the supply of field current to increase the field magnetic flux. Therefore, if the power cable is disconnected from the output terminal of the alternator due to an unexpected accident, so that the no-load saturation voltage appears at the output terminal, the voltage controller cuts the supply of the field current to decrease the field magnetic flux.
Usually, even after the supply of field current is cut, an inductance component exists in the field winding, which causes an excess high voltage to be produced (i.e., abrupt emission of magnetic energy), which may cause damage to the voltage controller. In order to avoid this problem, a circulating diode, as shown in FIG. 13, is used which does not attenuate the field current instantaneously when the supply of the field current is cut off, but works to circulate the field current through a closed circuit formed by the field winding and the circulating diode and converts the field current into thermal energy to attenuate it.
The current flowing at that time may be expressed quantitatively as
Vbxe2x88x92Vq=Lxc2x7dIf/dt+Rxc2x7If (during supply of field current t less than t0)xe2x80x83xe2x80x83(1) 
xe2x80x83xe2x88x92Vd=Lxc2x7dIf/dt+Rxc2x7If (after stop of supply of field current t greater than t0)xe2x80x83xe2x80x83(2)
If=(I0+Vd/R)xc2x7exp(xe2x88x92Rxc2x7(txe2x88x92t0)/L)xe2x88x92Vd/R(t greater than t0)xe2x80x83xe2x80x83(3) 
where L is the inductance of the field winding, R is the resistance value of the field winding, Vd is the forward voltage drop of the circulating diode, I0 is the value of the field current immediately before the supply of the field current is stopped, and Vq is the voltage drop when a power transistor working to supply and cut the field current is closed.
The behavior of the field current after the supply of the field current is stopped is expressed by a broken line b in FIG. 2(a). Specifically, the field current decreases at an exponential rate down to a final value Ifinal=xe2x88x92Vd/R when a time constant xcfx84=L/R. However, it is impossible for the current to flow through the circulating diode in the reverse direction, so that the current flow stops when a current value I(t) reaches zero (0). Specifically, even when the power transistor is opened to stop the supply of the field current, the field current continues to flow, thereby resulting in generation of the overvoltage for a period of time, as expressed by xe2x88x92(L/R)xc2x71n(Vd/(Vd+Rxc2x7I0)) when If=0 in Eq. (3).
Particularly, small-sized alternators in recent years tend to have a small resistance and a great inductance of the field winding, thus resulting in an increased duration of the overvoltage.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide an automotive alternator designed to attenuate the field current within a decreased period of time.
According to one aspect of the invention, there is provided an automotive alternator which comprises: (a) a rotor equipped with a plurality of field magnetic poles; (b) a first field winding working to magnetize the field magnetic poles; (c) a second field winding working to magnetize the field magnetic poles, the second field winding having a time constant shorter than that of the first field winding; (d) an armature subjected to a rotating field produced by the rotor to generate an alternating current output; (e) a rectifier converting the alternating current output from the armature into a direct current output; (f) a controller working to regulate field currents supplied to the first and second field windings to control an output voltage of the alternator; and (g) a circulating circuit connected in parallel to the first and second field windings, the circulating circuit working to circulate the field currents in the event that supply of the field currents is cut by the controller. For instance, if an excessive output voltage is developed in the event of a power cable is disconnected from the alternator, and supply of the field currents is cut, the field current flowing through the second field winding having a shorter time constant attenuates within a short period time, so that a counter-electromotive force decreases to zero (0). This causes the field current flowing through the first field winding to flow subsequently through the circulating circuit and the second field winding, so that the current having an opposite polarity flows through the second field winding, thereby attenuating magnetic fluxes intersecting the armature rapidly to eliminate the overvoltage of the alternator within a short period of time.
In the preferred mode of the invention, the circulating circuit includes a first circuit formed by a diode and a second circuit equipped with a circuit element working to enhance attenuation of the field currents. The alternator includes a selector which selects one of the first and second circuits. The controller works to control the selector to select the second circuit when a voltage of the output of the rectifier exceeds a reference value and select the first circuit when the voltage of the output of the rectifier is less than the reference value.
The second circuit is made up of a plurality of diode connected in series.
The rectifier is made of a rectifying circuit element implemented by a Zener diode. The controller selects through the selector the second circuit when the voltage of the output of the rectifier exceeds the reference value that is smaller than a zener voltage of the rectifying circuit element.
The controller switches the first circuit to the second circuit when a closed circuit including the first and second field windings is formed.
The first and second field windings are disposed coaxially.
According to another aspect of the invention, there is provided an automotive alternator which comprises: (a) a rotor equipped with a plurality of field magnetic poles; (b) a field winding supplied with a field current to magnetize the field magnetic poles; (c) an armature subjected to a rotating field produced by the rotor to generate an alternating current voltage; (d) a rectifier converting the alternating current output from the armature into a direct current output; (e) a circulating circuit including a first circuit formed by a diode and a second circuit equipped with a circuit element working to enhance attenuation of the field current, the circulating circuit being connected in parallel to the field winding to circulate the field current in the event that supply of the field current to the field winding is cut; and (f) a controller working to regulate the field current supplied to the field winding to control an output voltage of the alternator. The controller selects the second circuit when a voltage of the output of the rectifier exceeds a reference value and selects the first circuit when the voltage of the output of the rectifier is less than the reference value.
In the preferred mode of the invention, the second circuit is made up of a plurality of diode connected in series.
The diodes are implemented by a normal diode and a Zener diode joined in series in opposite directions.
The second circuit is made up of a diode and a resistor jointed in series with the diode.
The rectifier is made of a rectifying circuit element implemented by a Zener diode. The controller selects the second circuit when the voltage of the output of the rectifier exceeds the reference value that is smaller than a reverse zener voltage of the rectifying circuit element.
The controller switches the first circuit to the second circuit when a closed circuit including the field winding is formed.